Stents are generally tubular medical devices which, for curing of various diseases generated by stenosis or occlusion of a blood vessel or other lumen in vivo, are left indwelling in the stenosed or occluded lesion so as to dilate the lesion and secure the lumen.
The stent is so configured that for insertion into a body from outside, the stent is small in diameter when inserted, is thereafter expanded to be greater in diameter in the target stenosed or occluded lesion, and holds the lumen as it is.
Stents, in general, are hollow cylindrical bodies formed by processing metallic wire or metallic pipe. The stent is mounted to a catheter or the like in the state of being reduced in diametral size, is inserted in vivo, and is expanded in a target lesion by some method to come into close contact with and be fixed on the inner wall of a lumen, thereby maintaining the lumen shape. Stents are classified into self-expandable stents and balloon-expandable stents, according to function and method of indwelling. A balloon-expandable stent is a stent which itself does not have an expanding function. Instead, after the stent is inserted into a target lesion, a balloon is located in the stent, and the balloon is dilated so that the stent is expanded (plastically deformed) by the expanding (dilating) force of the balloon to come into close contact with and be fixed on the inner surface of the target lumen. This type of stent needs the just-mentioned stent-expanding work.
The purpose of indwelling of a stent is for preventing and restraining restenosis which might otherwise occur after such a procedure as PTCA is conducted. In recent years, use has come to be made of stents on which a physiologically active substance is carried and the physiologically active substance is locally released for a long period of time at the indwelling lesion in a lumen, for the purpose of lowering the restenosis rate. For instance, Japanese Patent Laid-Open No. Hei 8-33718 (EPC Publication No. EP0623354) discloses a stent in which a coating of a mixture of a therapeutic substance and a polymer is provided on a surface of a stent body. Besides, Japanese Patent Laid-open No. Hei 9-56807 proposes a stent in which a medicine layer is provided on a surface of a stent body and, further, a biodegradable polymer layer is provided on the surface of the medicine layer.
In addition, JP-T-2003-503152 (PCT Patent Publication No. WO01/001888) discloses a combined body of a plurality of stent pieces which are matched to one another, are aligned in a substantially annular shape and, further, are self-expanded in the radial direction. In this stent, the adjacent stent pieces are interlinked (coupled) to each other by at least one interlinking element formed from a plastically deformable material so that a flexible and limited motion is realized between the adjacent stent pieces. Besides, it is also disclosed that the interlinking element is formed from a degradable (decomposable) material.
Intensive and extensive investigations by the inventors here has led to the discovery that the blood vessel dilation maintaining force (strength) possessed by a stent may be one of the causes of restenosis. A stent having a lower blood vessel dilation maintaining force, however, cannot offer a sufficient improvement of the stenosed lesion of a blood vessel when left indwelling.